1. Field of the Invention
The present invention relates to a high frequency coil which is constructed to provide a shield case and a jar-like core both having through holes for adjusting inductance and is designed to prevent a core from being broken by a burr left on the edge of the hole of the through case.
2. Description of the Prior Art
A conventional high frequency coil is normally constructed to cover a bobbin having a coil wound thereon with a jar-like, core and a metal case with the threaded core being screwed into the bobbin. Such a high frequency coil provides through holes in the case and the jar-like core to provide access for a driver used to vertically move the threaded core.
It is better to make those holes as small as possible from the standpoint of a magnetic shield effect and a closed magnetic path effect.
To keep the quality against a magnetic flux focused on the edge of the hole of the case, the conventional high frequency coil provides a larger hole in the case than in the jar-like core.
However, the smaller coil results in disadvantageously breaking the jar-like core.
The present inventors studied and found the cause of this problem.
That is, as shown in FIG. 6, which is a sectional view showing holes of the metal case 40 and the jar-like core 43, a burr 42 whose length is about 0.1 to 0.2 mm is likely to be formed on the edge of a hole 41 of the case 40 when hole 41 is formed by means of press working.
As the overall dimension of the high frequency coil designed for surface mounting on a printed circuit board becomes smaller and smaller, the dimension of the jar-like core, one of the components of the coil, is made smaller. In particular, the thickness of the jar-like core is as thin as about 0.5 mm.
The jar-like core 43 is automatically pressurized and fitted into the case 40 by means of the machine. In this operation, the burr 42 formed on the edge 45 of the hole 44 of the core 43 may often break or crack a part of the edge 45, resulting in breaking the core 43.
It is possible to take the working step of removing the burr 42 from the case 40. However, this step contributes to increasing the cost of the case so that completed coil becomes too expensive to sell.